Making a color transparent in an image

I'm going to start off with my problem as making the image background transparent is the best solution I've come up with.

I have a large image that when saved as a PNG (so that I may have a transparent background) is ~1.3mb. Saved as a JPG it is around 300kb.

Obviously what I'm trying to do is to get a transparent background while also having a quick download.

My best solution to this was to set the background color to something specific then run the image through a filter and change the color to alpha 100%. This may take even longer than downloading the 1.3mb image itself with most high speed connections these days.

So far I can not figure out how to do this in GWT. I have passively searched for a solution for about a week. I can't seem to get from point A to point B with this problem.

I am open to alternative solutions to the problem or assistance with turning a specific color in an image transparent.